1. Field of the Invention
The present invention relates to the technical field of oil drilling, and more particularly to that of drilling rigs of the "jack-up" type, i.e. with the rig raised on legs, using a rack-and-pinion system, after being towed to the site.
2. Description of the Related Art
Such "off-shore" rigs are very well-known and consist schematically of the drilling rig or "hull" proper, which will not be described in detail here, generally mounted on three "legs", generally arranged in a triangle, and each provided with a base forming a supporting foot ("spud can" of the leg). The characteristic of these structures is to be assembled and then towed in a lowered floating state to the drilling site. They are then positioned on the site, and the rig is raised to the "high" position, i.e. to its operational height above the sea. Elevation is carried out by motorized displacement ("jacking") of the rig along each of its legs through "wells" arranged for the purpose. Each displacement along a leg is naturally independent of the others to a certain extent, so that the operator can, for example, correct uneven penetration of the legs into the sea bed. The rig is raised or lowered in relation to each leg by sets of racks-and-pinions mounted on each leg, the systems generally being arranged in each angle or "comer" of the triangular structure with three metal chords and struts ("chord" designating each "member" of each triangular leg). When the position of a given leg is changed, all the pinions of the system relating to this given leg are always in operation simultaneously, in the same direction and with the same theoretical speed. With regard to the speed of linear displacement, on the other hand, this depends on the load, which is one of the problems solved by the invention: when a leg is inclined, the least-loaded chord will be raised more quickly than the others by its motors, hence an additional increase in the RPD [Rack Phase Difference] on the leg concerned. The relative position of the rig (hull) and of the legs (geometry of the system) in relation to the sea bed is checked in relation to two series of fixed reference points, which are the bottom ofthe hull of the rig and the top of the "jacking structure" or "jacking house".
This positioning is, however, rendered difficult by many factors. These factors are also well known. An inclination or deviation may be due in the first instance to incorrect positioning by the operator, or to the existence or appearance of major lateral stresses or loads, such as those due to currents, swell and/or wind, or in particular to uneven embedding of the feet of the legs, or to heterogeneous or inclined ground, or to an operating fault on a lifting motor or brake. However, during operation, it is practically impossible to determine which factors are involved.
A fault of this type is manifested in particular in excessive stresses on the structure and particularly on the legs, with the risk of damage to the legs, to the structure of the leg, which can lead to an accident, and thereby to protracted immobilization of the rig, and to an appreciable reduction of the service life of the rig.
One concern of the industry is therefore to reduce this horizontality defect, referred to as "RPD" or "Rack Phase Difference" (difference for a given rack phase, i.e. geometrical difference in the structure for a given number of rack notches).
Normally, when the rig is on site, the legs are lowered until their tips are resting on the sea bed, then the rig is raised out of the water up to its operating position. This involves a certain penetration of the tips into the sea bed, according to the nature of the bed, but normally the legs remain vertical and the only forces acting on the unit remain within the strength limits selected when designing the rig. However, in certain cases, the sea bed may be inclined or uneven, etc., which can cause a deviation of one or more legs in relation to the vertical, which creates a bending moment on the leg concerned. Naturally, these deviations, if they affect two or three legs, are not necessarily "parallel", which complicates the problem posed. In its turn, such bending causes the load to become unequal on the three chords of the leg concerned, the leg being in a skewed position in its guide ("cock"). The imbalance can be such that it is no longer possible to move the rig. Corrective action is then necessary.
The rig then has to be lowered again to water level, to a floating state to eliminate the load, the supporting legs are withdrawn from the sea bed over a part of the penetration obtained, and then the jacking operation is recommended. This may possibly be combined, according to the seriousness of the situation, with a slight shift of position in order to avoid the first footprints, although such an operation is not generally recommended, and with a backward and forward movement of the legs ("reaming") to correct the deviation. Such action is obviously time consuming and is not always successful. In certain severe situations, a decision has to be made to move the rig from the planned drilling point to another location 50 to 100 meters away, and to recommence the operation, with the same uncertainties. This latter solution is impossible to implement when the rig has to be located alongside a fixed production platform. In such a case, only a margin of a few tens of centimetres is available for jacking up the rig.
It is therefore easy to appreciate the problem faced by the industry, which is to simplify such corrective action and to render it more efficient. The problem is aggravated by the magnitude of the stresses and mechanical forces involved, and by factors such as swell, wind, currents and the like, which can thwart the efforts of the operator. The problem is further complicated by the fact that an action which is as simple and reliable to implement as possible has to be proposed, since any inaccuracy on the part of the operator can cause major damage to the structure.